Mpg123 is a fast, free and portable MPEG audio player and decoder library mainly for Unix (but also ported to other platforms). It supports MPEG 1.0/2.0/2.5 layers 1, 2, and 3, with CD quality playback even on relatively slow CPUs.

AeonWave is a cross platform, hardware accelerated, multi-threaded, and versatile stereo and 4D audio mixing library. By combining hardware accelerated rendering and a low memory footprint the library can handle almost any situation, from virtual synthesizer apps for smart phones to immersive simulation. It has a small, easy to use, fault tolerant, and consistent API, low latency support by using hardware acceleration, simultaneous support for stereo and 4D audio, audio frames with sub-mixing capabilities, support for stereo and 3D audio effects and filters, and a Freeware supplemental OpenAL implementation.

beaTunes is an iTunes companion app capable of
automatically analyzing your music collection to
help you build better playlists. It analyzes BPM
(beats per minute), silence at the beginning or
end of a song, and the color (based on frequency
spectrum) of a given song. Some of the data is
stored through iTunes, and some is stored by
beaTunes itself. beaTunes can suggest albums that
would complement your music collection, display
music charts, and help you find and fix errors in
your song meta data, like typos and wrong genres.
It also allows you to blog about selected songs or
playlists.

Podget is a simple podcast aggregator optimized
for running as a scheduled background job (i.e.
cron). It features support for downloading
podcasts from RSS and XML feeds, for sorting the
podcasts into folders and categories, for
importing podcast URLs from iTunes PCAST files and
OPML lists, automatic M3U and ASX playlist
creation, and automatic cleanup of old podcasts.
It also features automatic UTF-16 conversion for
podcasts hosted on Windows servers and includes the ability to handle authentication for premium podcasts.

gPodder is a Podcast receiver/catcher written in Python and pyGTK. It manages podcast feeds for you, and automatically downloads all podcasts from as many feeds as you like. If you are interested in Podcast feeds, simply put the feed URLs into gPodder and it will download all episodes for you automatically. If there is a new episode, it will get it for you. It supports download resume, if the server supports it.

cmus is a small, fast, and powerful console music player for Linux and *BSD. It supports almost all common file formats, SHOUTcast/Icecast streaming, and multiple output plugins. cmus features multiple media library views, a playqueue, a directory browser, powerful filters, and vi-style search and keybindings. It also features gapless playback, ReplayGain, Unicode support, and customizable color schemes.